"I did not believe what he was saying, I did not believe parts of it and I thought, when I first met him, he was looking for something,” Retired NYPD Detective Thomas Burns said of John Avitto. (Jesse Ward/for New York Daily News)

The lead detective on the "Grid Kid Slayer" investigation did not believe the prosecutor's key witness 10 years ago.

Retired NYPD Detective Thomas Burns testified at a hearing on Friday where John Giuca's legal team intends to prove that John Avitto's trial testimony was fabricated.

Avitto, 46, testified at the 2005 murder trial for Fairfield University football player Mark Fisher that he overheard a conversation between Giuca and an older man — assumed to be Giuca's father — where he grilled him about the murder weapon.

"(Avitto) had a conversation with John Giuca later that day about the conversation with his father where his father asked 'Why did you have a gun in the house?' and he replied 'I don't know I just did,'" said Burns.

Burns, who met Avitto for the first time at a mental health facility in East New York three months after he allegedly eavesdropped on Giuca's conversation, took his story with a grain of salt.

"Did you believe John Avitto?" asked Giuca's attorney Mark Bederow.

"No," Burns replied and elaborated his response by adding, "I did not believe what he was saying, I did not believe parts of it and I thought, when I first met him, he was looking for something."

The legal team of John Giuca, pictured, intends to prove that Avitto's trial testimony was fabricated. (Jesse Ward/for New York Daily News)

Burns also testified he did not believe Avitto's testimony that Giuca was present for the shooting.

Avitto testified this week that prosecutor Anna-Sigga Nicolazzi helped him stay in substance abuse programs instead of going back to jail, whenever he violated the conditions, in exchange for his testimony against Giuca.

In January, Brooklyn prosecutors announced they were standing by the headline-grabbing guilty verdict against Giuca. The district attorney's Conviction Review Unit examined the contentious case after a lawyer for Giuca filed a 75-page document, alleging manufactured evidence and unsavory tactics were used to obtain a conviction, and that some witnesses were recanting their testimony.

But the review concluded there was nothing wrong with the finding that Giuca — who held himself as the head of a gang called "Ghetto Mafia" — killed the popular Fairfield University football player together with Anthony Russo.